(a) Field of the Invention
The invention relates to diffusion testing in which the result is indicated by a change in growth and/or other reaction. Such testing includes tests which may be biological (i.e. microbiological, immunological, serological and other biochemical tests inclusive of clinical chemical tests) or chemical as long as they are suitable for diffusion testing.
(b) Description of the Prior Art
Microbiological and immunological tests are often carried out by depositing an article for testing sensitivity, usually in the form of paper discs, tablets, or similar bodies, containing biologically active substances upon the surface of a solid or semi-solid substrate or medium. In the case of microbiological testing, the biologically active substance reacts with a culture of microoriganism inoculated on or in the substrate or with various substances in the substrate produced by the microorganism, whereas for immunological testing the active substance reacts with various substances, e.g. a serum added to the substrate or with various substances produced by a microorganism.
The substances deposited are diffused downwardly into the solid or semi-solid substrate to react with the aforementioned agents in the substrate, i.e., the microorganism or substance produced by the microorganism in the case of microbiological testing, and the added substance or substance produced by a microorganism in the case of immunological testing.
A common test of this type is the determination of sensitivity of microorganisms, e.g. bacteria, to biologically active substances, e.g. antibiotics, chemotherapuetics, etc. These experiments are frequently carried out by inoculating the substrate with the bacterial culture to be determined. Then the biologically active substance, i.e. an antibiotic contained in an article in the form of a paper disc or tablet, is deposited on the substrate, either manually or mechanically, and then pressed lightly into contact with the substrate with a sterile needle, tweezers or the like. The disc or tablet is absorbent and the solid and semi-solid substrate contains aqueous medium which is absorbed in the disc or tablet whereupon the antibiotic is dissolved and diffuses into the substrate. The substrate is then placed in an incubator and, after sufficient time, the effect of the antibiotic is noted by measuring the diameter of the circular zone of inhibition of the growth (i.e. region of sensitivity) surrounding the article. Mention should also be made of the use of biologically active substances which stimulate or enhance growth, i.e. of growth factors and growth requirements, such as vitamins, amino acids, peptides, proteins, carbohydrates and minerals.
Various other examinations which involve diffusion and change in growth and/or other reaction in a solid or semi-solid substrate e.g. radial immunodiffusion, can also be performed by the above method. Additionally, since diffusable agents from the substrate contained in the aqueous medium will diffuse into the article, it is also possible to perform examinations which involve reaction in the article.
In order to compare the test results of bacterial sensitivity to various antibiotics, or the other biologically active substances, it is essential that uniform and good contact between the article containing the biologically active substance and the substrate is obtained. It is also important to achieve as uniformly adjusted contact as possible from one sensitivity test to the next in order to compare the results and establish a basis for reliability.
A frequent source of error in determining sensitivity occurs when the article containing the biologically active substance is deposited or placed askew on the substrate. Because of incomplete contact between the article containing biologically active substance and the substrate, diffusion into the substrate will be impaired since the amount of diffusion is a function of the contact area. Good contact results when the intended contact surfaces are in substantially complete and uniform contact thus resulting in suitable and reproducible amounts of diffusion.
If uniformly good contact between the article containing the biologically active substance and the substrate containing the microorganism is not achieved, diffusion of the biologically active substance into the substrate will not be uniform and consequently a standard diffusion test procedure cannot be developed, since diffusion is highly dependent on the size of the contact area. The results of sensitivity tests determine to a large extent how patients with infections diseases shall be treated.
Heretofore, it was impossible manually to achieve a uniformly good contact between biologically active substance and the bacteria-containing substrate or medium from one sensitive test to the next. Since laboratory technicians vary in manual dexterity, proficiency, and work experience, it is not surprising that uniformly good contact cannot be achieved from one sensitivity test to another. Even greater variations in results can be expected between different laboratory technicians.
Although many attempts were made to overcome the foregoing and other difficulties none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.